Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
407 PM akst Wednesday Nov 26 2014
Analysis and upper levels...
there is a fairly sharp upper level ridge over western/central
Alaska. There is north-northwest flow aloft over eastern Alaska on
the east side of this ridge. To the west there is a deep upper low
over the central and western Aleutians. An upper trough and
surface low associated with this upper low is over the northern
Bering with a weakening front just west of the alaskan West Coast.
A strong surface low centered just west of Adak is rapidly moving
north. The front associated with this low stretches from the south
central Bering to the southeast across the aleutains just west of
Umnak Island...into the northern Pacific where it bends back
toward the south-southwest.
the numerical models are in very good agreement through about 72
hours (12z saturday) then in fairly good agreement to 84 hours.
There is still some spread in the location of a low moving toward
the Aleutians in the 72-84 hour time frame...but less than in
previous runs. Forecast confidence is a little above normal for
the afternoon package.
Short term forecast southcentral Alaska (days 1 and 2)...
The main story through Friday will be the strength of outflow gap
winds across the northern Gulf Coast...and areas of patchy fog and
low stratus around Cook Inlet...the Copper Basin...and the
Matanuska Valley. Outflow gap winds are expected to peak tonight
as the pressure gradient tightens ahead of the building ridge. The
strongest gap winds will be across Prince William Sound into
Valdez...although they will not be all that impressive as a
whole...generally remaining at or below gale force. Patchy fog and
low stratus will be a concern across Cook Inlet into the
Matanuska Valley as lingering clouds get trapped along the
western facing mountains...with weak low level flow promoting
inversions and moistening from the Knik Arm and Cook Inlet. There
is low confidence on the exact placement and density of the
fog...but locally dense fog will be possible at times tonight and
tomorrow morning...especially from Anchorage to the mat valley.
Elsewhere...a weak front will move into the southwest Gulf Friday
morning...spreading rain and possibly mixed rain and snow from
Kodiak to the southern Kenai Peninsula coast.
Short term forecast southwest Alaska...
high pressure over the southwest slowly pushes off to the east as
a front moves over the area Thursday morning from the Bering. This
front will bring primarily snow to the area that will likely mix
with rain along the coast. However...with warmer air overrunning
some of the more protected areas of colder air at the surface there
is a possibility of some light freezing rain around Bristol Bay
and the Kuskokwim Delta.
Short term forecast Bering Sea/Aleutians...
the low over the southern Bering will continue its track north
leaving westerly flow in its wake for the Bering and Aleutians.
This flow will bring in colder air that will start to change the
rain to snow starting in the western Aleutians. These showery
conditions will last into the weekend until the next system comes
from the North Pacific bringing in more steady rain.
Long term forecast (days 3 through 7...Saturday through
wednesday)...this time frame will feature several more broad
areas of low pressure over the Bering eventually pushing towards
south central by the beginning of next week. The first of these
lows will move north into the Bering Thanksgiving day. Cold air
wrapping around it on Friday will dig a trough into the North
Pacific south of Bristol Bay. This trough should provide upper
level support...albeit weak...to a weak front moving through south
central on Saturday. Modestly deep southwest flow should coincide
with this support to finally push some moisture into the area.
This will likely be the best chance for snow for many areas that
we have seen for weeks. While models are still ironing out some of
the critical details...a couple of inches of accumulation does
look possible for the Anchorage area with higher amounts possible
over higher elevations. This system will be a quick-hitter though
as it moves to the alcan border by Sunday morning.
Early Sunday another low will bull its way from the North Pacific
into the western Bering. It will bring another round of gale force
winds to much of the central and western Aleutians Sunday into
Monday. It will also push a fairly Sharp Ridge toward the
Mainland. This ridge will keep the pattern rather progressive and
should limit snow accumulations from the afore mentioned front.
Models are now in decent agreement that the low will pivot over
the western Bering as it taps into some colder air off of Russia.
This cold air advection could tighten the gradient enough to make
for a decent swath of storm force winds with possibly hurricane
force gusts over the western Aleutians. Meanwhile...the low will
produce a massive warm occlusion that will head towards Bristol
Bay late Sunday and into south central by late Monday. With
several weak upper level waves accompanying this feature and warm
air overriding colder air at the surface...this will make for a
very messy pattern with possible mixed precipitation types.
The parent low rejoins the 100kt+ jet over the North Pacific and
tracks along the Aleutians through Tuesday. This will serve to
keep areas from the eastern Aleutians through south central rather
warm and moist through Wednesday.
Marine...gales 127 170 172 173 174 175 176 177 178 179 181 185 411
412 413 414.
Synopsis and model discussion...bl
southwest Alaska/Bering Sea/Aleutians...dek